Radiation system and lithographic apparatus

ABSTRACT

A radiation system for generating a beam of radiation that defines an optical axis is provided. The radiation system includes a plasma produced discharge source for generating EUV radiation. The discharge source includes a pair of electrodes constructed and arranged to be provided with a voltage difference, and a system for producing a plasma between the pair of electrodes so as to provide a discharge in the plasma between the electrodes. The radiation system also includes a debris catching shield for catching debris from the electrodes. The debris catching shield is constructed and arranged to shield the electrodes from a line of sight provided in a predetermined spherical angle relative the optical axis, and to provide an aperture to a central area between the electrodes in the line of sight.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase of PCT/NL2007/050598, filedNov. 27, 2007, which claims benefit and priority to U.S. applicationSer. No. 11/637,936, filed on Dec. 13, 2006. Both priority applicationsare hereby incorporated in their entirety by reference.

FIELD

The present invention relates to a radiation system and a lithographicapparatus that includes a radiation system.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning” direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In addition to EUV radiation, radiation sources used in EUV lithographygenerate contaminant material that may be harmful for the optics and theworking environment in which the lithographic process is carried out.Such is especially the case for EUV sources operating via a plasmaproduced discharge source, such as a plasma tin source. Such a sourcetypically comprises a pair of electrodes to which a voltage differencecan be applied. In addition, a vapor is produced, for example, by alaser beam that is targeted to, for example, one of the electrodes.Accordingly, a discharge will occur between the electrodes, generating aplasma, and which causes a so-called pinch in which EUV radiation isproduced. In addition to this radiation, the discharge source typicallyproduces debris particles, among which can be all kinds ofmicroparticles varying in size from atomic to complex particles, whichcan be both charged and uncharged. It is desired to limit thecontamination of the optical system that is arranged to condition thebeams of radiation coming from an EUV source from this debris.Conventional shielding of the optical system primarily includes a systemcomprising a high number of closely packet foils aligned parallel to thedirection of the light generated by the EUV source. A so-called foiltrap, for instance, as disclosed in EP1491963, uses a high number ofclosely packed foils aligned generally parallel to the direction of thelight generated by the EUV source. Contaminant debris, such asmicro-particles, nano-particles and ions can be trapped in wallsprovided by the foil plates. Thus, the foil trap functions as acontamination barrier trapping contaminant material from the source. Dueto the arrangement of the platelets, the foil trap is transparent forlight, but will capture debris either because it is not travellingparallel to the platelets, or because of a randomized motion caused by abuffer gas. It is desirable to improve the shielding of the radiationsystem, because some (directed, ballistic) particles may still transmitthrough the foil trap.

SUMMARY

According to an aspect of the invention there is provided a radiationsystem for generating a beam of radiation that defines an optical axis.The radiation system includes a plasma produced discharge sourceconstructed and arranged to generate EUV radiation. The discharge sourceincludes a pair of electrodes constructed and arranged to be providedwith a voltage difference, and a system constructed and arranged toproduce a discharge between the pair of electrodes so as to provide apinch plasma between the electrodes. The radiation system also includesa debris catching shield constructed and arranged to catch debris fromthe electrodes, to shield the electrodes from a line of sight providedin a predetermined spherical angle relative the optical axis, and toprovide an aperture to a central area between the electrodes in the lineof sight.

According to an aspect of the invention, there is provided alithographic apparatus that includes a radiation system for generating abeam of radiation that defines an optical axis. The radiation systemincludes a plasma produced discharge source constructed and arranged togenerate EUV radiation. The discharge source includes a pair ofelectrodes constructed and arranged to be provided with a voltagedifference, and a system constructed and arranged to produce a dischargebetween the pair of electrodes so as to provide a pinch plasma betweenthe electrodes. The radiation system also includes a debris catchingshield constructed and arranged to catch debris from the electrodes, toshield the electrodes from a line of sight provided in a predeterminedspherical angle relative the optical axis, and to provide an aperture toa central area between the electrodes in the line of sight. Thelithographic apparatus also includes a patterning device constructed andarranged to pattern the beam of radiation, and a projection systemconstructed and arranged to project the patterned beam of radiation ontoa substrate.

Other aspects, features, and advantages of the present invention willbecome apparent from the following detailed description, theaccompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a schematic first embodiment of a radiation system of thelithographic apparatus of FIG. 1 according to an aspect of theinvention;

FIG. 3 shows schematically a second embodiment according to an aspect ofthe invention;

FIG. 4 shows a further embodiment according to an aspect of theinvention;

FIG. 5 shows a modification of the arrangement described with referenceto FIG. 4;

FIG. 6 shows an alternative modification of the arrangement describedwith reference to FIG. 4;

FIG. 7 schematically shows a deflection principle of debris from the EUVsource;

FIG. 8 schematically shows a quadrupole magnet arrangement for providingdebris deflection;

FIGS. 9 a-c illustrate a further embodiment of the arrangement of FIG.4;

FIG. 10 shows a graph related to a thermal cleaning of the radiationsystem;

FIG. 11 shows an embodiment of the thermal cleaning principle referredwith respect to FIG. 10;

FIG. 12 shows another embodiment of the thermal cleaning principlereferred with respect to FIG. 10;

FIGS. 13 a-e show embodiments of continuous and droplet fluid jets;

FIG. 14 shows a schematic perspective view of a radiation systemaccording to an embodiment of the invention;

FIG. 15 shows a schematic perspective view of a cross section of theradiation system of FIG. 14;

FIG. 16 shows a schematic perspective view of a wiping module of aradiation system according to an aspect of the invention;

FIG. 17 shows a schematic top view of the wiping module of FIG. 16;

FIG. 18 shows a schematic cross-sectional side view of the wiping moduleof FIG. 16;

FIG. 19 shows a schematic cross-sectional side view of a wiping moduleof a radiation system according to another aspect of the invention;

FIG. 20 shows a schematic perspective view of a wiping module of aradiation system according to a further aspect of the invention;

FIG. 21 shows a schematic cross-sectional side view of a radiationsystem according to an embodiment according to the invention; and

FIG. 22 shows a diagram of collectable optical power as a function of anopening semi-angle of a debris catching shield.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises: an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or EUV radiation); a support structure (e.g. a mask table)MT constructed to support a patterning device (e.g. a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g. a wafer table) WT constructed to hold a substrate (e.g. aresist-coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters; and a projection system (e.g. a refractive orreflective projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g. comprising one or more dies) of the substrate W.

The illumination and projection systems may include various types ofoptical components, such as refractive, reflective, diffractive or othertypes of optical components, or any combination thereof, for directing,shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, or any combination thereof, as appropriate forthe exposure radiation being used. Any use of the term “projection lens”herein may be considered as synonymous with the more general term“projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery systemcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator and acondenser. The illuminator may be used to condition the radiation beam,to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF2 (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor IF1 can be used to accurately position themask MA with respect to the path of the radiation beam B, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the mask table MT may be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) themask table MT may be connected to a short-stroke actuator only, or maybe fixed. Mask MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks as illustrated occupy dedicated targetportions, they may be located in spaces between target portions (theseare known as scribe-lane alignment marks). Similarly, in situations inwhich more than one die is provided on the mask MA, the mask alignmentmarks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

In FIG. 2 a schematic first embodiment is shown of a radiation systemaccording to an aspect of the invention. In particular, there is shown aradiation system 1 for generating a beam of radiation 2 in a radiationspace. The radiation space is bounded by a predetermined spherical anglerelative to an optical axis 3. The radiation system 1 includes a plasmaproduced discharge source 4 for generating EUV radiation. The dischargesource includes a pair of electrodes 5 that are constructed and arrangedto be provided with a voltage difference, and a system that typicallyincludes a laser 6 for producing a vapor between the pair of electrodesso as to provide a discharge 7 between the electrodes 5. It has beenfound that debris 8 coming from the radiation system 1 is primarilyproduced on or near the electrodes 5. These effects also cause ageneration of a so-called pinch which develops between the electrodes 5.Typically, the EUV light that is generated is produced by an electrontransition in a Tin atom (or another suitable material, for example,Lithium or Xenon), which is ionized multiple times of electrons in thedischarge process. It was found that debris particles 8, in particular,ballistic particles of the kind that may contaminate the downstreamoptics, are mainly produced on or near the electrodes 5 in debrisproducing zones 9, where the central EUV source light is mainly producedin the pinch zone 10 that is distanced from the debris producing zones9. Thus, for a plasma produced discharge source 4, the debris producingzones 9 are typically distanced from the EUV radiation producing pinchzone 10. This effect can be utilized by the illustrated embodiment,which according to an aspect of the invention comprises a shield 11 toshield the electrodes 5 from a line of sight provided in a predeterminedspherical angle relative the optical axis 3 and to provide an aperture12 to a central area between the electrodes in the line of sight.Accordingly, debris 8, which is generated in the debris producing zone 9initially (in the absence of additional electromagnetic fields, however,see the embodiment illustrated in FIG. 5-FIG. 7) travels substantiallyin straight lines from the zone 9. Thus, a shield 11 that shields theelectrodes 5 from a line of sight in a predetermined spherical anglearound the optical axis 3 is able to trap these debris particles 8, sothat in the line of sight a substantial amount of debris 8 is preventedfrom entering downstream optics (not shown). Additionally, the shield 11substantially does not shield the radiation coming from the EUVradiation producing pinch zone 10, since it provides an aperture 12 to acentral area (conforming to a designated pinch zone 10) between theelectrodes 5 in the line of sight, which accordingly can travel into thedownstream optics substantially unhindered by the shield 11. In thisway, the debris (which comes from the electrodes) may be stopped by theshield, without stopping the EUV radiation. Practically, it isconvenient to shield both electrodes, since it is probable that bothelectrodes generate debris-producing zones that can attribute in debris8 production.

The shielding effect can be further optimized by placing the shields 11close enough, preferably, a distance ranging between 0.5 and 25 mm toany of the electrodes, to shield a maximum spherical angle of the debrisproducing zone 9.

To minimize a distance with the electrodes, the heat load will be sohigh on the shield 11 that it is preferably provided as a fluid jet 13,for example, of molten Tin. Such a jet could have a length of about 75mm and a thickness of several mm, for example ranging from 0.5 to 3 mm.It is noted that fluid jets are per se known from US 2006-0011864 whichdiscloses electrodes in a plasma discharge source in the form of fluidjets, however, there is not disclosed a shield or at least one fluid jetprovided near an electrode of a pair of electrodes. Accordingly,preferably, the debris catching shield 11 is provided, as illustrated,by a pair of fluid jets 13, arranged oppositely and generally parallelto a longitudinal axis of the electrodes 5. It may however, in certainembodiments, possible to direct the plasma production substantiallytowards one of the electrodes 5, which one electrode will accordingly bea major contributor in producing debris 8. Such debris may vary in sizeand travel speed. For instance, one can have micro-particles: these aremicron-sized particles with relatively low velocities. In addition,there may be produced nano-particles, which are nanometer-sizedparticles with typically quite high velocities; atomic debris, which areindividual atoms that act as gaseous particles; and ions, which areionised high-velocity atoms.

It is noted that in one embodiment, the fluid jet 13 may be providednear an electrode of the pair of electrodes without substantially beingconfigured to shield the electrodes from a line of sight provided in apredetermined spherical angle relative the optical axis and to providean aperture to a central area 10 between the electrodes in the line ofsight (unlike the embodiment shown in FIG. 2). In such an embodiment,according to another aspect of the invention, the fluid jet 13 may beaccelerating the recombination rate of the plasma, which may increasethe frequency of the EUV source 4 and accordingly may provide a higherpower output of the radiation system. Specifically, the fluid jet 13 maycomprise molten Tin, although other materials may be feasible to providethe same recombining effect, including, for example water or a liquidgas, such as liquid nitrogen or liquid argon. An advantage of the latteris that it may evaporate and thus may leave no further traces in thesystem. Additionally, the fluid is preferably of an electricallyconductive material and may be kept at ground potential, although othermaterials, such as argon and nitrogen may also be used.

The advantage of the fluid jets is that the obstruction is continuouslyreplaced and can thus withstand very high heat loads. However, in otherembodiments, it may be possible, to provide a shield 11 that ispositioned at generally the same distance nearby the electrodes 5 asdiscussed hereabove with reference to FIG. 2, but that is not formed bya fluid jet, but by a moving element (not shown), for example, anaxially moving metal strip, that moves generally parallel to anelectrode longitudinal axis, and which may be cooled by providingcoolant in a container, for guiding the moving element there through.

FIG. 3 shows schematically an embodiment of the invention, showing ashield in the form of a plurality of fluid jets 13, arranged in radialdirection relative from the central area 10 between electrodes 5 in theline of sight. In such an embodiment, the fluid jets 13 are providedadjacent to each other, and may be generally aligned to form a staticconfiguration of generally radially oriented platelets 14, relative tothe central area 10. Although within the general context of theinvention, preferably, these platelets are oriented to shield theelectrodes 5 from a line of sight provided between the platelets 14,this embodiment may also have practical applications with the plateletsoriented to include the electrodes 5 in a line of sight provided betweenthe platelets 14. These applications may benefit from the heat loadcapacity of the shield 11 that is provided by the fluid jets 13. Afurther advantage is that the jets 13 by nature are not contaminated bydebris depositions since they are continuously renewed. This is incontrast with a conventional foil trap solution where solid platelets 14(foils) are used to provide shielding from debris 8. These conventionalplatelets therefore may suffer from contamination which may hinder aproper transmission of the EUV radiation.

In particular, especially for plasma produced discharge sources 4operated with Tin plasma, a suitable material for the fluid jets mayalso be Tin or a compound comprising Tin, such as for example Ga—In—Sn,which may be suitable to have a lower melting point and easier handlingproperties. Furthermore, although FIG. 3 shows an embodiment wherein thejets 13 are dimensioned with a general circular form, other form,including strip forms may be feasible, thus providing a shield 11comprising platelets 14 in the form of single jets, generally of theform as depicted in FIG. 4. A thickness of such liquid foil may betypically 0.5-1 mm, which is slightly thicker than conventional foilthicknesses that are about 0.1 mm thick. It is noted that thin liquidfoils are discussed in T. Inamura, H. Tamura, H. Sakamoto,“Characteristics of Liquid Film and Spray Injected from swirl CoaxialInjector”; Journal of Propulsion and Power 19 (4), 623-639 (2003). Inthis publication, cone-shaped foils are produced. However, preferably,according to an aspect of the invention, a slit-shaped nozzle is used,in particular, for providing straight-formed jets that are radiallyoriented relative to a centre zone 10 wherein a pinch can develop. Inaddition, this static embodiment may be combined with a rotating foiltrap, known per se from EP1491963 and, of course, with other embodimentsdescribed in the current document.

Under certain circumstances, fluid jets may not be stable—i.e. they mayspontaneously divide into droplets with a diameter approximately equalto the jet diameter. This means that it may only be possible to createcontinuous jets if the diameter is relatively large (>˜0.5 mm).Therefore, it may be advantageous to use jets that intentionally consistof closely spaced droplets that can have a very small and controllablesize, with a controllable distance between droplets. The ability tocreate such stable droplet chains (40 μm diameter with about 40 μmdistance) was presented in the EUVL Sematech conference in Barcelona(Conference 7870, 17 Oct. 2006) by David Brandt (session 3-SO-04) foruse as a laser target in a LPP EUV source.

The stability of the droplet chains means that different configurationsmay be employed, depending upon which functional aspects (recombinationand/or debris catching) need to be optimized. FIGS. 13 a-e show examplesof such configurations. FIG. 13 a depicts a continuous jet 13 in whichthe recombination surface is moving in the direction T. FIG. 13 bdepicts a stable train of droplets 113, moving in direction T, which forthe purposes of this invention may be considered to be a jet 13. Thestability of the droplet chains means that these chains may bepositioned adjacent to each other to add an extra degree of flexibilitywhen implementing the invention. FIG. 13 c shows two adjacent chains ofdroplets 113, effectively creating a jet 13, extended in one directioncompared to the jet 13 of FIG. 13 b. A disadvantage of a droplet chainis that debris has a possible path to pass through the fluid jet. FIG.13 d and FIG. 13 e show how the droplet chains can be shifted in thedirection of movement T with respect to each other to effectively createa virtual continuous jet 13 for debris having a trajectory in the planeof the figure and perpendicular to the direction of movement T of thejet.

FIG. 4 in addition shows a further embodiment according to an aspect ofthe invention, wherein the debris catching shield, herebelow alsoindicated as a foil trap 15 comprises a static configuration ofgenerally radially oriented platelets 14, relative to the central area10, wherein the platelets 14 are oriented to shield the electrodes 5from a line of sight provided between the platelets 14. In thisembodiment, at least some of the platelets are of a solid nature, inparticular, of foils used in a so called conventional foil trap. It isnoted that WO 99/42904 A1 discloses a foil trap of generally the sameconfiguration; however, the publication does not discuss that theplatelets 14 are configured to shield the electrodes 5 from a line ofsight provided in a predetermined spherical angle relative the opticalaxis and to provide an aperture to a central area 10 between theelectrodes in the line of sight. In comparison with conventionalrotating foil traps of the type as disclosed in EP1491963, this staticfoil trap configuration may have an advantage in easier coolingproperties, since, in an embodiment, this static foil trap configurationcan be cooled using static coolant circuits devised on or in proximityof the platelets 14. Since the configuration is static, accordingly,cooling may be much simpler and therefore, the configuration can beeasily scaled to higher power levels of the source. In addition, thisconfiguration has as a benefit that it does not require moving parts,which may provide constructional advantages since the required strengthand dimensions of the platelets 14 may be of a different order than therotating conventional construction, which requires complex parts such asair bearings and high tension materials that can withstand centrifugaltension forces applied to the platelets. Thus, according to the proposedembodiment, the radially oriented platelets 14 are aiming at the pinchzone 10 thus substantially unhindering transmittance of EUV-radiation16. This foil trap 15 will fill up with debris at certain locations so aslow rotation around the optical axis (e.g. once a day) could be usefulto make sure no debris will contaminate the next foil trap 15 or otheroptics. This may be useful, since in a preferred embodiment, the opticalaxis may be 45 degrees with respect to a level plane. This principlecould also be designed in combinations of concentric circles and plates.In addition, the geometry of the depicted embodiment, including staticradially oriented platelets 14, may have stacking dimensions that havehigh gas resistance wherein a distance between the platelets may be inan order of 0.5-2 mm, preferably about 1 mm. Accordingly, atomic debrismay be trapped easier. Also, a high gas resistance may help to allow alower buffer gas pressure near the pinch zone 10, which may resulting ina higher efficiency EUV power. Typically, such a buffer gas may be Argongas.

In addition to the thermal cleaning techniques illustrated withreference to the FIG. 10-FIG. 12 presented herebelow, the platelets 14may provided as a material of porous characteristics for removing thedebris from the platelets through capillary action. For instance, byusing foil material with porous characteristics (e.g. sinteredmaterials) Tin can be taken out of the optical path and drained (orbuffered in an exchangeable element). Accordingly, lifetime of thedebris suppression system may be increased and downtime due to foil trapcleaning may be minimized.

In addition to the above-discussed cleaning technique, the radiationsystem may comprise an excitator 17 (see FIG. 4) for removing the debrisfrom the platelets 14 through mechanical excitation of the platelets 14.For example, by rotating the module fast enough (˜2000-3000 RPM as anindication) on a temporarily basis, the tin may be spun of the relevantfoils, and may be caught by a getter 18. In an embodiment, therevolution axis is the optical axis, but other axes of revolution mayalso be possible. A combination of rotation and vibration is also anoption. Accordingly, the excitator may comprise a centrifuge forremoving the debris from the platelets through centrifugal action andadvantageously a getter 18 for catching debris 8 removed from theplatelets.

Also, the foil could be externally excitated (longitudinal waves) so aflow of tin in a predefined direction may be present. Also (directional)accelerations/vibrations can be used to give excitation profile(s)(pending between stick/slip effect of the droplets) to the entire moduleinstead of each separate foil.

FIG. 5 discloses a further embodiment of the arrangement described withreference to FIG. 4. In this embodiment, a deflecting electromagneticfield unit 19 is disposed between the electrodes 5 and a shield, in thisembodiment illustrated as foil trap 15. By applying an electromagneticfield, charged debris particles 8 traveling from the debris producingzones 9 can be deflected, which accordingly can be used to virtuallyexpand the distance between the EUV radiation producing pinch zone 10and the debris producing zones 9 as will be made even more clear withreference to FIG. 7. In FIG. 5, the deflecting field is produced by apair of electrodes 20 arranged oppositely to the optical axis.Accordingly, a static electric field is generated according to which theelectrically charged particles can be deflected.

In FIG. 6, in contrast to the embodiment depicted in FIG. 5, or inaddition to it, the electromagnetic deflecting field is provided as astatic magnetic field 21, due to magnet elements 26 (see FIG. 8)arranged around the optic axis 3. For a front view of thisconfiguration, see FIG. 8. Although various static field configurationsare feasible, an optimally defined field is provided as a quadrupolefield, arranged for deflecting substantially all electrically chargedparticles 8 traveling generally in a direction towards the opticalsystem (not shown), towards a plane 22 oriented along the radiallyoriented platelets 14 and generally parallel to a length axis of theelectrodes 5. Preferably, as is also shown in the Figure, this plane 22is provided along the optical axis 3. However, it may be possible toselect another region that is more off axis to deflect the particlesthereto. Accordingly, charged debris particles can be deflected moreeasily towards the platelets 14 of the foil trap 15, which virtuallyincreases the distance between the electrodes 5. Consequently, fewerplatelets 14 may be needed to achieve a given extent of debrissuppression. Accordingly, a typical distance may range between 0.5 and 3mm, preferably about 2 mm. This significantly increases the opticaltransmission of the foil trap.

The principle of operation in FIG. 6 is as follows. The rectangle 10indicates an acceptance width of the foil trap in the absence of amagnetic field and is accordingly generally corresponding to a zone 10from where EUV radiation is produced. However, particles 8 generatednear the edges of the zone 10 (accordingly, produced from a debrisproducing zone 9) may travel unhindered through the shield, in thisembodiment illustrated as foil trap 15, without being intercepted, asillustrated by the trajectory 23.

By applying a magnetic field of the type as indicated (with aconventional arrow indication), such debris particles 8 are deflectedtowards the optical axis 3. For example, the particle with trajectory 23may be deflected to follow the solid line 24 and no longer betransmitted through the foil trap 15. This is because on entrance of thefoil trap, the particle appears to originate from a point outside theacceptance width 10 as indicated by the other dashed line 25. In otherwords, the application of the magnetic field effectively narrows down aneffective acceptance width of the shield, which width defines a zonefrom where debris particles could enter the system unhindered.Accordingly, for a given dimensioning of the acceptance width, theoptical transmission may be improved by reducing the number of platelets11 and applying a magnetic field.

A typical distance for the acceptance width of the foil trap in theabsence of a magnetic field may be ranging from about 0.5 to about 2 mm,preferably about 1 mm. For typical foil trap dimensions (inner radius 30mm, relative to a central zone 10, outer radius 139 mm), this leads to afoil trap with 137 foils having an optical transmission of approximately63%. As the Figure shows, in a preferred embodiment, the distance d, d′between the platelets 14 may vary, wherein typically a distance dtowards the optical axis 3 may increase relative to distances d′ awayfrom the optical axis 3.

FIG. 7 shows how the source of the particles, that is, the debrisproducing zone 9 can be virtually shifted over a distance d to a virtualdebris producing zone 9′ by applying the magnetic field. Accordingly, aneffective acceptance width may be reduced.

In the presence of a magnetic field B, a particle with charge q andvelocity v experiences a Lorentz force given by

F=qv×B  (1)

Consequently, if the direction of the magnetic field is perpendicular tothe velocity, the particle follows a circular trajectory with radius Requal to

$\begin{matrix}{R = \frac{mv}{qB}} & (2)\end{matrix}$

In the present embodiment, the angular deflection α due to the magneticfield depends on the distance over which the field is applied, which isapproximately equal to the inner radius of the foil trap r₀. Thedeflection angle is given by sin α=r₀/R as shown in FIG. 3. The apparentpoint of departure of the particle is accordingly displaced over adistance d given by

d=r ₀ sin α−R(1−cos α)  (3)

which for small values of α reduces to

$\begin{matrix}{d = \frac{r_{0}^{2}}{2\; R}} & (4)\end{matrix}$

By substituting Eq. (2), the following expression relating thedisplacement d to the characteristic parameters q, m and v of the debrisparticles is obtained:

$\begin{matrix}{d = \frac{{qBr}_{0}^{2}}{2\; {mv}}} & (5)\end{matrix}$

Using permanent magnets or electromagnets, a magnetic field of the orderof 1 T can fairly easily be achieved. When a magnetic field is appliedso that the displacement d is equal to 0.5 mm for a certain type ofdebris, the acceptance width for that debris accordingly effectivelydecreases by a factor of 2 compared to the earlier mentioned value of 1mm acceptance width. One can therefore construct a foil trap that has anacceptance width of 2 mm and still obtain the same extent of debrismitigation. Such a foil trap may have only 69 foils and an opticaltransmission of 70%. Thus, the optical transmission is significantlyimproved by applying a magnetic field.

FIG. 8 shows a front view, seen along the optic axis, of the electrodes5 and a quadrupole magnet configuration of magnets 26. In thisconfiguration, the North-South lines of opposing magnets 26 are orientedalternating and generally parallel to the longitudinal axis of theelectrodes 5. Accordingly, a magnetic field may be produced that followsthe orientation depicted in FIG. 6, that is, with a general direction ofthe magnetic field on either sides of the optic axis 3 in a planegenerally parallel to the length axis of the electrodes, to deflect theparticles inwards towards a plane 22 coaxial with the optic axis 3.Accordingly, for typical configurations, positively charged particlesare focused to a vertical plane (by focusing in the horizontal directionand spreading in the vertical direction). Alternatively, a similar (butless well-defined) deflecting field may be obtained by placing twoidentical magnetic poles on opposite sides of the optical axis.

FIG. 14 shows a schematic perspective view of a further embodiment of aradiation system 1 according to an aspect of the invention. Theradiation system 1 is arranged for generating a beam of radiation in aradiation space. FIG. 15 shows a schematic perspective view of a crosssection of the radiation system 1 of FIG. 14. Similar to the radiationsystem shown in FIG. 2, the radiation system 1 shown in FIGS. 14 and 15comprises a plasma produced discharge source for generating EUVradiation. The discharge source includes a pair of electrodes 5 that areconstructed and arranged to be provided with a voltage difference, and asystem that typically includes a laser for producing a vapor between thepair of electrodes 5 so as to provide a discharge between theelectrodes. Further, the electrodes 5 define a discharge axis 40interconnecting said electrodes 5. The discharge axis 40 traverses thecentral area between the electrodes. The radiation space issubstantially bounded between two mutually reversely oriented cones 41,42 relative to the discharge axis 40, the cones 41, 42 having their apex43 substantially in the central area between the electrodes 5. The twocones 41, 42 have a diabolo type appearance. The radiation system 1further comprises a debris catching shield constructed and arranged tocatch debris from said electrodes 5 from a line of sight provided in theradiation space 44 bounded between the two cones 41, 42, and to providean aperture to the central area between the electrodes in said line ofsight. The debris catching shield extends circumferentially around thedischarge axis 40 over at least 180°, preferably over at least 270°. Byarranging the shield such that the shield surrounds the discharge axis40 over at least 180° the effective optical output of the plasma sourceis relatively high. A beam of radiation generated by the plasma sourceand passing the debris catching shield has a larger spherical extensioncompared with the embodiment of the radiation system shown in FIG. 2. Asa consequence, the performance of the plasma source output that can becollected for further processing, increases with respect to theradiation system shown in FIG. 2. Further, by extending the debriscatching shield circumferentially around the discharge axis 40 up to360° an optimal effective optical output is obtained. In one embodiment,the shield extends over a circumferential range of approximately 270° toapproximately 360°, a space near the discharge axis is available, e.g.for inspection purposes and/or for arranging devices, such as a systemfor producing the vapor between the pair of electrodes and/or a coolingstructure.

The debris catching shield of the radiation system 1 in FIG. 14 includesa ring shaped or ring section shaped structure that is substantiallyrotationally symmetric with respect to the discharge axis 40. As aconsequence, debris suppression can be obtained along radial directionsin a substantial circumferential range around the discharge axis 40,viz. in a circumferential range of at least 180° around the dischargeaxis 40. The debris catching shield comprises a static configuration ofgenerally radially oriented platelets, relative to the discharge axis40, wherein the platelets are oriented to shield the electrodes from aline of sight provided between the platelets. It appears that gooddebris suppression can be obtained along directions having an angle ofat least 45° with respect to the discharge axis 40. Platelets of thedebris catching shield have concentric conical surfaces and/or compriseat least one planar section.

In a preferred embodiment, the platelets, also called foils, haveconcentric conical surfaces aligned with respect to the discharge axis40, with their apex at the central area along the discharge axis. Inanother embodiment, the foils can be composed of a multiple number ofplanar sections, the foil being aligned with respect to the dischargeaxis. For example, each foil may have, in cross-sections thereof, ahexagonal or octagonal shape.

FIG. 9 shows a further embodiment of the static configuration ofgenerally radially oriented platelets 14 described with reference toFIG. 4. In this embodiment, instead of solid monolithic platelets 14, inat least some of the platelets 14, traverses 27 are provided orientedgenerally transverse to the platelets 14. This embodiment may providethermal isolation to the further downstream platelets 14, as seen fromthe EUV source 4. In addition to it, possibly by applying fluid jets asshown in FIG. 3, preferably on a proximal side of the platelets 14relative to the EUV source 4, the heat load to the platelets 14 can befurther managed. In addition, a gas 28 can be guided through thetraverses 27 of the platelets 14, which may be used for cleaningpurposes of the platelets 14, for example, a hydrogen radical gas.Accordingly, the platelets 14 can be cleaned to prevent debrisdepositing on the platelets 14, thereby preventing a situation in whichEUV light will no longer be able to pass through the platelets.Preferably, the foil trap may be cleaned without having to take the foiltrap out of the system. The principle of additional traverses in theshown foil trap embodiment could also be used for other types of foiltraps, in particular, in non-static foil traps.

In addition to, or alternatively, the traverses may be used as a buffergas to provide a buffer gas zone within a zone in side the platelets, inorder to be able to further trap, for example, neutral nanoparticleswhich may diffuse through the platelets 14 and may cause contaminationof the optical system provided downstream (not shown). FIG. 9A shows aside view of an embodiment with traverses 27, which may be provided withalternating use of wires 29 and platelet parts 30.

FIG. 9B shows an embodiment with only wires 29; to provide aconfiguration similar to the fluid jet configuration depicted in FIG. 3.FIG. 9C in addition shows a top view generally seen along an axisparallel to the length axis of the electrodes 5, of the plateletembodiment depicted in FIG. 9A. The more open structure of FIG. 9B hasan advantage when integrating foil trap cleaning based on hydrogenradicals, because it becomes easier to bring the reactive H radicals tothe surface of the foils, and it becomes easier to transport thereaction products out of the foil trap 15. However, the drawback is thatthe flow resistance of the foil trap 15 becomes lower, which may make itmore difficult to achieve a high buffer gas pressure. Therefore oneneeds to optimize the amount of openings in the platelets. The preferredembodiment therefore is in most cases a partially open foil structure,as shown in FIG. 9A. Furthermore, in a preferred embodiment H cleaningis integrated with the wired structures shown in the figures byproviding an electric current supply 31, which is connected to at leastsome of the wires 29 of a platelet 14. At least some of the wires 29 inthe platelet are now interconnected in order to allow a current to runthrough several wires 29 simultaneously. With a high enough current (forexample, 20 A for a 0.4 mm thick wire), the wires will form a filamentthat will reach temperatures of about 2000° C. where typically H2molecules will dissociate, generating H radicals. These H radicals canthen react with Sn to form gaseous SnH4, which is pumped out of thesystem. In order to add H2 to the system, the embodiment thereforefurther comprises a H2 gas inlet 32 and the embodiment comprises avacuum pump 33 to remove gas from the system (as shown in FIG. 9C).

Alternatively, it is possible to remove debris from the capture shieldusing evaporation. FIG. 10 schematically indicates a comparison betweenremoval rates by evaporation for lithium and tin. Along the horizontalaxis is plotted the temperature, in degrees Celsius. Along the verticalaxis is plotted the removal rate (nm/hour). In particular FIG. 10 showsa graph of a calculation that was performed to calculate the removalrate of tin and lithium, for temperatures in a range of 200-800° C. Inaddition, for tin a removal rate of about 0.1 nm/hour was calculated fora temperature of about 900 K, and a rate of about 1E5 nm/hour for atemperature of about 1400 K, with an almost exponential increase. Thus,in a range between these temperature values, by providing a heatingsystem (which may be EUV source 4) the debris catching shield, inparticular a foil trap 15 of the kind as shown in FIG. 4 may beselectively heated to elevate a temperature of the debris shield to atemperature for evaporating debris from the debris catching shield. Inaddition a gas supply system is provided which may in use serve forproviding a buffer gas flow between the platelets, and which may offline be used for cleaning purposes, in particular, for providing a gasflow to evacuate evaporated debris from the debris catching shield. Aparticular preferable elevation temperature of the debris catchingshield for a tin plasma source may be at least 900 K for offlinecleaning purposes. Accordingly an alternative may be provided forchemically reactive cleaning, which may be harmful to the optics system.For a temperature of the platelets 14 of 940 K (667 C) a Tin evaporationof 0.4 nm/hour may be achievable.

Advantageously, a lithium plasma source is used since lithium has asignificantly higher vapor pressure than tin (about 9 orders ofmagnitude) and as a consequence also a significantly higher removal rate(removal rate of 0.4 nm/hr requires temperature of only 550 K (277 C).This allows applying evaporative cleaning of lithium-contaminatedsurfaces at significantly lower temperatures than evaporative cleaningof tin-contaminated surfaces; evaporative cleaning of collector shellscontaminated with lithium is feasible.

FIG. 11 shows a general schematic illustration of the cleaning principleexplained hereabove with reference to FIG. 10. In particular, a platelet14 is heated, so that debris 8 deposed thereon will be evaporated. Byproviding a gas flow 34 along the platelet 14, the evaporated debris,for example, tin vapor 35, will be carried away from the platelet,through which the platelet can be cleaned. Although FIG. 11 has beenexplained with reference to a gas flow along a platelet 14 of a foiltrap, the cleaning principle can be used generally, to clean EUV mirrorsurfaces in particular, of downstream optical elements such as acollector element.

In FIG. 11, the object to be cleaned (a platelet 14 or mirror optic) isheated while a gas is flowing over the mirror in order to transport thetin vapor away from the mirror. Heating can be done with a heatingdevice, but it is also possible to temporarily reduce active cooling ofthe object, and use the heat generated by the EUV source.

In FIG. 12 this technique is used for the collector 36 of an EUVlithography setup. In this embodiment the collector shells are heatedone-by-one, in order to evaporate the tin from the reflective side ofthe collector shell, and to deposit the tin vapor on the backside of thecollector shell below. When a collector shell 37 is heated, it willtypically evaporate tin on both sides of the shells. This means thatalso the backside of the shell will evaporate tin and deposit this onthe reflective surface of the collector shell above. To prevent this itis preferable to heat the center shell first, and then continue with thenext shell, etc. Thus by cleaning the collector shells in the rightorder and controlling the temperature of the collector shell at the sametime, it is possible to minimize (re)deposition on the reflectivesurface.

FIG. 16 shows a schematic perspective view of a wiping module 60 of aradiation system according to an aspect of the invention. The wipingmodule 60 is provided with a multiple number of substantially paralleloriented wiping elements 61 that are movable along respective plateletsurfaces 62 of a debris catching shield. FIGS. 17 and 18 show furtherschematic views of the wiping module 60, in a top view and across-sectional side view, respectively. A single frame supports thewiping elements 61. In particular, the wiping module is implemented as acomb-like structure wherein the individual wiping elements 61 form thefingers of the comb. The width of the wiping elements 61 are chosen suchthat the elements 61 fill an intermediate space 63 between adjacentplatelet surfaces. As consequence, platelet surfaces arranged oppositeto each other can be wiped simultaneously by performing one or moremovements of the wiping module 60 with respect to the surfaces to becleaned. A local wiping element width W substantially equals theintermediate space 63 distance between two adjacent platelet surfaces.It is noted that in another embodiment according to an aspect of theinvention, the wiping elements are not oriented substantially parallel,but otherwise, e.g. mutually deviating arranged for following a surfaceshape of a platelet to be cleaned.

By moving the wiping module 60 along a moving path of a wiping element61 with respect to a platelet surface 62 contamination particles, suchas Sn contamination, is swept and/or pushed away from the plateletsurface 62. As the spacings between platelets of the debris catchingshield, also called foil trap, can be small, contamination particlesmight quickly fill said intermediate spacings, thereby strongly reducinga transmittance of the foil trap. This is especially the case with foiltraps that are directly exposed to micro particle debris emitted by anEUV source, such as described referring to FIGS. 14 and 15. Thus, byapplying the wiping module 60, contamination particles can be removedfrom the platelet surfaces, thereby improving the transmittance of thefoil trap.

As the wiping elements 61 are substantially parallel oriented, similarlyoriented platelet surfaces can be cleaned. Further, instead of using asingle frame for supporting the wiping elements, multiple supportingelements can be used for supporting the wiping elements. It is alsopossible to mutually interconnect ends of the finger-like wipingelements, thereby obtaining a plate-like structure having slots forreceiving the platelets.

It is noted in this context that during use of the wiping module, thewiping elements move with respect to the platelet surfaces, meaning thatthe wiping elements move, or the platelets, or both such that a netrelative movement results. Along a moving path of the wiping elementswith respect to the platelet surfaces, the intermediate spacing distancebetween two opposite platelet surfaces remains substantially constant,thereby maintaining an efficient wiping operation. In an alternativeembodiment, the distance between the opposite platelet surfaces alongsaid path varies, e.g. for providing locally a low sweep resistance forthe movement of the wiping elements.

The wiping elements 61 are arranged for performing a translation and/orswiveling movement with respect to the respective platelet surfaces 62.In the embodiment shown in FIGS. 16-18, the wiping elements 61 perform atranslation, i.e. the elements 61 move in a moving direction M,substantially transversely with respect to the plane wherein the wipingelements extend. The platelets 62 are substantially planar. Further, theplatelet structure of the debris catching shield, the foil trap, issubstantially invariant in the moving direction M, thereby allowing anefficient cleaning operation of the wiping module 60. The movingdirection M is substantially transverse with respect to both a dischargeaxis and an optical axis of the source.

Viewed from the discharge 7 between the electrodes of the source, theplatelets 62 extend substantially between a fixed radial inner distanceand a fixed radial outer distance, see e.g. FIG. 18. As can be deducedfrom the figures, some space is to be reserved for accommodating thewiping module 60 between the source and a collector, especially when thewiping elements 61 are at end positions of the moving path, in FIG. 18at an uppermost and lowermost position.

Depending on an accumulation rate of contamination particles, e.g. Sn,the wiping module 60 can be moved along the platelet surfaces of thefoil trap at a specific time interval, e.g. once every 5 minutes. Thiscan be done online during operation of the source. However, asignificant amount of radiation can be blocked during a wiping actionand it may be necessary to compensate this loss of illumination with alonger illumination time, e.g. using a feedback system with a dosesensor. In a non-operational state of the wiping module 60, in astationary position, the wiping module is preferably placed outside acollection angle of the source, in order to counteract any radiationblocking. As an example, the wiping module can in the non-operationalstate be placed in an uppermost or lower position. Alternatively, thewiping module may be placed on the optical axis, the position as shownin FIG. 18, so that it is optimally aligned with source radiation paths,so that optical losses are relatively small.

In an embodiment according to an aspect of the invention, the wipingmodule further comprises one or more wipers 64 that are positioned toclean the wiping elements 61 from contamination particles that arecollected during a wiping movement. Preferably, the wiping module alsocomprises a collection base 65 to collect the contamination particlesthat are removed from the wiping elements. As shown in FIG. 18, thewipers 64 can be positioned to clean the wiping elements when the moduleis in its uppermost position or in its lowermost position.Alternatively, the wipers can also be positioned for cleaning the wipingelements in either the uppermost position or lowermost position. In theembodiment shown in FIG. 18, the wipers 64 perform a movement along thesurface of the wiping elements 61. By collecting the contaminationparticles in the one or more collection bases 65, the particles such asSn, can be removed, e.g. for re-use. In another embodiment according tothe invention, the wiping elements 61 are arranged for moving along astationary wiper 64, see e.g. FIG. 19 showing a schematic cross-sectionview of a wiping module embodiment. Specifically, the wiper mightcomprises two wiper sections placed opposite with respect to each otherand defining a receiving opening for receiving the wiping elements 61.In further embodiments according to an aspect of the invention, thewiping elements are cleaned otherwise, e.g. by using a hydrogen orhalogen cleaning or evaporation process.

FIG. 20 shows a schematic perspective view of a wiping module 60 of aradiation system according to a further aspect of the invention. Here,the platelets 14 of the foil trap are curved, in particular theplatelets have concentric conical surfaces aligned with respect to adischarge axis of the source as explained referring to FIG. 14. The apexof the platelets are located substantially at a central area along thedischarge axis. In the embodiment shown in FIG. 20, the wiping elements61 of the wiping module 60 are arranged for performing a swivelingmovement with respect to the respective platelet surfaces. The swivelingaxis of the swiveling movement substantially coincides with thedischarge axis of the EUV source. Since the spacing between theplatelets is substantially invariant under swiveling with respect to thedischarge axis, an effective and efficient wiping operation can beperformed. In radiation system shown in FIG. 20 a more compactconstruction is obtained. In particular, no substantial additional spaceis required for the wiping module in a non-operational state. Further,the wiping elements block merely a minimum amount of radiation duringoperation as the wiping elements are always aligned with the centralarea between the electrodes. In addition, the cleaning process atextreme positions of the wiping elements becomes easier.

According to a further aspect of the invention, the surface of thewiping elements is treated for enhancing its wetting properties, e.g. byreduction of oxides or by applying a coating.

It is noted that the described wiping module variants can also beapplied in combination with other debris catching shield types. As anexample, such a wiping module can be applied in combination with adebris catching shield that extends circumferentially around thedischarge axis over at least 180°, preferably over at least 270°,optionally over 360°. In such an embodiment, the debris catching shieldcan be rotated with respect to the discharge axis, thereby performing acleaning action by means of a stationary wiping module. Therefore,according to an aspect of the invention, a radiation system is providedfor generating a beam of radiation in a radiation space, the radiationsystem comprising a plasma produced discharge source constructed andarranged to generate extreme ultraviolet radiation, the discharge sourcecomprising a pair of electrodes constructed and arranged to be providedwith a voltage difference, and a system constructed and arranged toproduce a discharge between said pair of electrodes so as to provide apinch plasma between said electrodes, a debris catching shieldcomprising platelets constructed and arranged to catch debris from saidelectrodes, and a wiping module provided with a multiple number ofsubstantially parallel oriented wiping elements movable along respectivesurfaces of said platelets. In a preferred embodiment according to anaspect of the invention, the intermediate distance between plateletsurfaces is substantially invariant along a moving path of a wipingelement with respect to a platelet surface to be cleaned.

FIG. 21 shows a schematic cross-sectional side view of a radiationsystem according to an embodiment according to the invention. Theradiation system 1 comprises a plasma produced discharge source and adebris catching shield as explained referring to FIGS. 14 and 15. Thesource includes a pair of electrodes 5 between which electrodes adischarge 7 is generated during operation of the radiation system 1. Ina radiation space that is bounded between two mutually reverselyoriented cones, a beam of radiation generated passing through a debriscatching shield having a static configuration of generally radiallyoriented platelets 14. In the shown embodiment, the platelets 14 form aring-shaped foil trap. Further, the system 1 comprises a collectorconfiguration for modifying a generated beam of radiation, wherein thecollector configuration substantially surrounds the plasma produceddischarge source in a circumferential direction around the dischargeaxis. The collector configuration comprises a normal incidence reflector44 that extends circumferentially substantially around the plasmasource. In FIG. 21, an upper cross section 44 a and a lower crosssection 44 b of the reflector 44 is shown. The reflector 44 is arrangedfor reflecting the beam of radiation passed through the foil trap. Inthe shown embodiment, the reflector 44 is provided with an ellipticreflector surface so that the beam 46 a, 46 b incident upon thereflector surface is transformed into a converging beam 48 a, 48 bpropagating towards an intermediate focus point 50. It is noted that thecollector configuration can be arranged to extend over a reducedcircumferential range, e.g. over a circumferential range ofapproximately 270° with respect to the plasma source, in particular ifthe debris catching shield also does not entirely enclose the dischargeaxis 40 in the circumferential orientation. Further, instead of applyinga single normal incidence collector, a grazing incidence collector, or acombination of a normal incidence collector and a grazing incidencecollector might be applied.

In addition, it is noted that a collector configuration substantiallysurrounding a plasma produced discharge source can not only be appliedin combination with a radiation system according to the invention havinga debris catching shield constructed and arranged to catch debris fromelectrodes of a plasma source, to shield said electrodes from a line ofsight provided in the radiation space, and to provide an aperture to acentral area between said electrodes in said line of sight, but also incombination with other radiation systems, e.g. provided with a rotatingfoil trap configuration. Therefore, according to an aspect of theinvention, a radiation system is provided for generating a beam ofradiation in a radiation space, the radiation system comprising a plasmaproduced discharge source constructed and arranged to generate extremeultraviolet radiation, the discharge source comprising a pair ofelectrodes constructed and arranged to be provided with a voltagedifference, and a system constructed and arranged to produce a dischargebetween said pair of electrodes so as to provide a pinch plasma betweensaid electrodes, and a collector configuration for modifying a generatedbeam of radiation, wherein the collector configuration substantiallysurrounds the plasma produced discharge source in a circumferentialdirection around discharge axis interconnecting said electrodes. In apreferred embodiment according to the invention, the collectorconfiguration extends circumferentially around the discharge axis overat least 180°, preferably over at least 270°, optionally over 360°. In afurther preferred embodiment according to the invention, the collectorconfiguration is substantially rotationally symmetric with respect tothe discharge axis between the electrodes.

FIG. 22 shows a diagram of collectable optical power as a function of anopening semi-angle of a debris catching shield. An amount of effective,collectable optical power transmitted through the debris catching shieldcan be calculated by subtracting the solid angle allocated to the cones41, 42 in FIG. 14 from a total of 4π. The solid angle subtended by asingle cone of opening semi-angle α is given by 2π (1−cos α). Hence, thetotal solid angle that can be collected is given by:

Ω=4π−4π(1−cos α)=4π cos α=4π sin θ  (6)

where θ is the opening semi-angle of the foil trap. For example, a foiltrap with θ=45° covers 71% of the total solid angle of 4π.

The amount of power that is actually transmitted through the debriscatching shield can be calculated by integrating the transmittance ofthe debris catching shield over the covered solid angle. Thetransmittance of the debris catching shield increases with 0 due to theincreasingly dense spacing between the foils.

FIG. 22 shows a diagram of collectable optical power as a function of anopening semi-angle of a debris catching shield. The diagram shows afirst curve 80 representing the collectable solid angle as a function ofthe semi-angle of the shield according to equation 6, assuming thatoptical power is emitted in 4π and that no losses occur in passing theshield. Further, the diagram shows a second curve 81 wherein opticallosses have been incorporated according to parameters of a typical foiltrap shield. From the diagram, it can be deduced that, as an example,using a foil trap with θ=45°, 45% of the radiation emitted in 4π can becollected after the foil trap. The diagram further shows a third andfourth curve 82, 83 representing a collectable power without and withlosses in the foil trap, respectively, in a typical radiation system asshown in FIG. 5, assuming a typical collection with respect to theoptical axis of a beam of radiation. As can be seen from the diagram,the amount of optical power that can be collected using a typicalring-shaped foil trap with θ=45° is about four times higher than thecollectable power in the typical radiation system collecting a beam ofradiation using a foil trap as e.g. shown in FIG. 5.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A radiation system for generating a beam of radiation in a radiationspace, the radiation system comprising: a plasma produced dischargesource constructed and arranged to generate extreme ultravioletradiation, the discharge source comprising a pair of electrodesconstructed and arranged to be provided with a voltage difference, and asystem constructed and arranged to produce a discharge between said pairof electrodes so as to provide a pinch plasma between said electrodes;and a debris catching shield constructed and arranged to catch debrisfrom said electrodes, to shield said electrodes from a line of sightprovided in the radiation space, and to provide an aperture to a centralarea between said electrodes in said line of sight.
 2. (canceled)
 3. Aradiation system according to claim 1, wherein the debris catchingshield comprises at least one fluid jet.
 4. A radiation system accordingto claim 3, wherein the fluid jet comprises molten tin or a tincompound.
 5. (canceled)
 6. A radiation system according to claim 1,wherein the debris catching shield is provided by a pair of fluid jets,arranged oppositely and generally parallel to a longitudinal axis of theelectrodes.
 7. A radiation system according to claim 3, wherein thedebris catching shield comprises a plurality of fluid jets, arranged inradial direction relative from the central area.
 8. (canceled) 9.(canceled)
 10. (canceled)
 11. (canceled)
 12. A radiation systemaccording to claim 1, wherein the debris catching shield comprises astatic configuration of generally radially oriented platelets, relativeto said central area, wherein the platelets are oriented to shield theelectrodes from a line of sight provided between said platelets.
 13. Aradiation system according to claim 12, wherein a distance between theplatelets is increased relative to distances away from the optical axis.14. (canceled)
 15. A radiation system according to claim 12, furthercomprising an electromagnetic deflecting field unit disposed forapplying an electromagnetic deflecting field between the electrodes andthe shield.
 16. A radiation system according to claim 15, wherein saidelectromagnetic deflecting field unit provides a static magnetic field.17. (canceled)
 18. (canceled)
 19. A radiation system according to claim12, further comprising a hydrogen radical supply system for guidinghydrogen radicals through said platelets.
 20. (canceled)
 21. (canceled)22. (canceled)
 23. A radiation system according to claim 12 wherein atleast some of the platelets are provided by a fluid jet.
 24. A radiationsystem according to claim 23, wherein the fluid jet comprises molten tinor a tin compound.
 25. (canceled)
 26. A radiation system according toclaim 1, further comprising a heating system that can be selectivelyheated for elevating a temperature of said debris catching shield to atemperature for evaporating said debris from said debris catchingshield; and a gas supply system for providing a gas flow to evacuatesaid evaporated debris from said debris catching shield.
 27. (canceled)28. (canceled)
 29. (canceled)
 30. A radiation system according to claim12, wherein the platelets are provided as a material of porouscharacteristics for removing said debris from said platelets throughcapillary action.
 31. A radiation system according to claim 12, furthercomprising an excitator for removing said debris from said plateletsthrough mechanical excitation of said platelets.
 32. (canceled) 33.(canceled)
 34. (canceled)
 35. A radiation system according to claim 1,wherein the system that is constructed and arranged to produce adischarge between said pair of electrodes comprises a laser.
 36. Aradiation system according to claim 1, wherein the electrodes define adischarge axis interconnecting said electrodes and wherein the radiationspace is substantially bounded between two mutually reversely orientedcones relative to the discharge axis, the cones having their apexsubstantially in the central area between the electrodes.
 37. (canceled)38. A radiation system according to claim 12, wherein platelets haveconcentric conical surfaces and/or comprise at least one planar section.39. (canceled)
 40. (canceled)
 41. A radiation system according to claim12, comprising a wiping module provided with a multiple number of wipingelements movable along respective platelet surfaces.
 42. (canceled) 43.(canceled)
 44. (canceled)
 45. (canceled)
 46. (canceled)
 47. Alithographic apparatus comprising: a radiation system constructed andarranged to generate a beam of radiation defining in a radiation space,the radiation system comprising: a plasma produced discharge sourceconstructed and arranged to generate extreme ultraviolet radiation, thedischarge source comprising a pair of electrodes constructed andarranged to be provided with a voltage difference, and a systemconstructed and arranged to produce a discharge between said pair ofelectrodes so as to provide a pinch plasma between said electrodes; anda debris catching shield constructed and arranged to catch debris fromsaid electrodes, to shield said electrodes from a line of sight providedin the radiation space, and to provide an aperture to a central areabetween said electrodes in said line of sight; a patterning deviceconstructed and arranged to pattern the beam of radiation; and aprojection system constructed and arranged to project the patterned beamof radiation onto a substrate.